Initiation of reverse transcription in HIV occurs on an RNA-RNA complex between HIV genomic RNA and human tRNALys, 3. Biochemical and genetic studies have revealed a possible secondary structure for the RNA complex, and large conformational changes in both RNAs have been proposed. The initiation complex is an obvious target of drug interven- tion, but such an approach requires high resolution structure determi- nation. The proposed research uses the methods of nuclear magnetic resonance spectroscopy (NMR) to gain structural information on the initiation complex and its component RNAs. Oligonucleotides that correspond to critical regions of HIV RNA and tRNALys, 3 will be synthesized and their structures determined at high resolution. In particular, blocking the conformational rearrangements that occur on complex formation may be a possible drug strategy. Larger RNA fragments that correspond to domains of the initiation complex will be studied by NMR. The intact initiation complex (150 nts) will be characterized using specific isotopic labeling strategies. Although the NMR structure determination will be difficult, the technologies to be applied are proven, and the initiation complex is unlikely to be crystallized in the near future. The results of the structure determination will be used for model building and biological assays in the context of the program project grant.